In U.S. patent application Ser. No. 226,562, filed Jan. 19, 1981, by Richard B. Sykes, et al., there is described a method for preparing the new antibacterial agent, [3S-[3.alpha.(Z)4.beta.]]-3-[[(2-amino-4-thiazolyl)[(1-carboxy-1-methyleth oxy)imino]acetyl]amino]-4-methyl-2-oxo-1-azetidinesulfonic acid, having the formula ##STR1## and hereinafter referred to also as aztreonam The compound so obtained is in the form of hydrated crystals, which have been designated as the .alpha.-form, which are relatively unstable.
In U.S. patent application Ser. No. 282,636, filed July 13, 1981, by David Floyd et al., there is described a method for recrystallizing the above-mentioned .alpha.-form of aztreonam from an anhydrous organic solvent to form the .beta.-form of aztreonam. The .beta.-form is anhydrous, substantially non-hygroscopic and more stable than the .alpha.-form.